The detectors considered in the present invention are known detectors, whether of the point or matrix type, and regardless of the materials, semiconducting or otherwise, of which they are composed. The signals emitted by these detectors can either be electric currents, or they can be of a physical nature and can be converted to electric current in a known manner. It will simply be assumed that reception of a particle by the detector triggers an output signal having the form of a pulse of a certain width and the maximum amplitude of which is representative of the energy of this particle.
In the rest of the description, for the purpose of clarity, reference will be made to the detection of “photons” (more particularly measurement of the characteristics of electromagnetic radiation), but it is to be borne in mind that the invention is completely independent of the nature of the particles detected.
An important factor limiting the quality of signal processing is the background noise which is always present in the current emitted by the detector. This background noise includes at least two components. The first component is the “dark current”, more particularly, the fluctuating current, of thermal origin, emitted by the detector even when it is not receiving any photons. The second component is the “transient decay current”, more particularly the fluctuating current that is manifested for a certain time after reception of a photon by the detector; in detectors using semiconducting materials, this transient decay current is due in particular to crystal imperfections in these materials.
Let us examine the effects of this background noise on the accuracy of measurements carried out by the known methods.
If for example a measurement system based on integration is used, by means of which the total energy of the radiation received by the detector during a predetermined time is measured, the current from the detector is integrated over this time. A faithful representation of the energy of the photons received then requires taking into account all of the current produced, including the low values: use of a threshold of detection of this current would therefore be detrimental, in that it would cause a loss of information. However, the measured current includes, as explained above, a component due to the background noise, to which an exact value cannot be ascribed because of the fluctuations due to thermal drift and to the transient decay current, and because of the random noise associated with this component. In the known systems, the current due to the background noise is integrated in the measured value, and then a quantity that is only an estimated average value of the effect of the background noise is subtracted, in order to obtain the representative value of the radiation energy.
As another example, when a measurement system based on counting is used, by means of which the number of photons of energy E above a threshold E2 received by the detector during a predetermined time is measured, a suitable device (for example a bistable circuit) is triggered when the signal exceeds a certain threshold value corresponding to E2, and the said device is reset when the signal falls below this threshold value. Admittedly, there is then nothing to prevent the placement, at the detector output, of a system for filtering the continuous component from the background noise. However, the problem is that it is not possible to distinguish an increase in current due to the arrival of a photon from the increase in current caused by a fluctuation of the background noise, unless the said threshold value is set high enough so that the fluctuations can practically never exceed it. In addition, in these conventional measurement systems, the bistable circuits or similar devices produce parasitic coupling. In practice, this threshold cannot therefore be set very low.